The present invention relates to ink jet printing devices and, more particularly, to a method and apparatus for controlling with a high degree of accuracy the deposit of ink drops on a print receiving medium.
Jet drop recorders, such as that shown in U.S. Pat. No. 3,701,998, issued Oct. 31, 1972, to Mathis, have included one or more rows of orifices which receive electrically conductive recording fluid, such as a water base ink, from a pressurized fluid manifold and eject the fluid in parallel fluid filaments. Mechanical stimulation is applied to the structure or fluid coupled to the orifices, causing the filaments each to disintegrate into a plurality of drops.
Graphic reproduction in recorders of this type has been accomplished by selectively charging some of the drops in each of the streams. The drops then pass through an electrical field which deflects the charged drops such that they strike a drop catching device. The uncharged drops pass unaffected through the deflection field however, and are deposited on a moving web of paper or other material. Although the direction of web movement has generally been substantially perpendicular to the row or rows of drop streams, web movement which is at an inclined angle with respect to the row or rows of drop streams has also been used to increase the effective drop density across the width of the web, as shown in U.S. Pat. No. 4,010,477, issued Mar. 1, 1977, to Frey, and assigned to the assignee of the present invention. Rows of drop streams inclined to the direction of web movement are also shown in "Ink Jet Head," by Krause, IBM Technical Disclosure Bulletin, Volume 19, Number 8, January 1977, pages 3216 and 3217.
One method of charging drops in a recorder is to apply charge control signals to charging electrodes positioned near the drop streams, adjacent the point at which the drops are formed. As the drops separate from their parent fluid filaments, they carry a portion of the charge induced by the charging electrodes. The static deflection field which separates the charged drops from the uncharged drops has been generated by applying a constant potential difference between one or more catchers and a deflection ribbon. U.S. Pat. No. 3,787,882, issued Jan. 12, 1974, to Cassill, discloses a thin deflection ribbon which is positioned between and parallel to two rows of parallel drop streams, with catchers positioned outwardly of the drop streams. Charged drops in each drop stream will be deflected away from the deflection ribbon and toward the appropriate catcher.
One problem with jet printers of this type has been attaining sufficient image resolution. Since a discrete number of drops are applied to form the printed images, it is clear that an increase in the number of drops deposited per unit area of print medium, and a corresponding increase in data handling capability, will permit improvement in image definition. If, however, only one print position per print line is serviced by each orifice, the number of drops per unit width and, therefore, the resolution of an image in the direction transverse to the print web, are limited by the minimum dimensions required between each orifice. The approach taken in the Mathis device is to provide two staggered rows of drop streams. Charging of the drops in the two rows is timed such that printing from the two rows of streams is in registration.
The head assembly disclosed in the Frey patent includes a number of rows of jet streams which are arranged along angularly oriented placement lines, thus increasing the effective density across the web. The head assembly shown in Krause combines these approaches by providing two interlaced rows of drop streams positioned obliquely with respect to the direction of web movement.
Another approach to the problem of resolution is shown in U.S. Pat. No. 3,373,437, issued Mar. 12, 1968, to Sweet et al and assigned to the assignee of the present invention. FIG. 6 of the Sweet reference shows a number of jets in a single row in a converging array. The interdrop spacing is less, therefore, than the water-jet spacing. The configuration is disadvantageous, however, in that the distance travelled by the drops in each stream will be slightly different, complicating the timing of data severely. Additionally, it is somewhat difficult to insure that the streams will continue to converge as they approach the web.
In U.S. Pat. No. RE 28,219, issued Oct. 29, 1974, to Taylor et al, and assigned to the assignee of the present invention, printing by separate ink jet printer arrays in a tandem relation, with each successive array being laterally offset, is shown. The orifices are positioned such that they interlace to provide print capability across the entire web.
Another approach taken to increase drop density has been to use a single jet to service a number of print positions across the print web. U.S. Pat. No. 3,739,395, issued June 12, 1973, to King, and assigned to the assignee of the present invention, discloses a printer in which uncharged drops are caught while the charged drops from each orifice are deflected by two sets of deflection electrodes to a plurality of discrete print positions on the moving web. Deflection of the drops is either perpendicular or parallel to the direction of web movement, or both, covering either a one line matrix or a multiple line matrix across the web. The minimum distance between jet orifices is somewhat greater in the King device than in previously mentioned devices, however, since deflection electrodes must be positioned on all sides of each orifice.
In U.S. Pat. No. 3,972,052, issued July 27, 1976, to Atumi et al, an ink jet printing device is disclosed in which a single jet scans a plurality of print lines in succession. Each scanning operation is controlled by two pairs of deflection electrodes which provide parallel deflection fields through which the ink drops pass in succession. Identical ramp deflection voltages are applied to the deflection electrodes, with the deflection voltage applied to the second pair of deflection electrodes being delayed with respect to the deflection voltage applied to the first pair. Drops are selectively charged in accordance with the data to be printed. The deflection potentials, while varying cyclically to effectuate the scanning of the print lines, are not varied in accordance with the print image data.
U.S. Pat. No. 3,871,004, issued Mar. 11, 1975, to Rittberg, discloses a printing head which moves transversely across a print web. Individual deflection electrodes are positioned adjacent each orifice to deflect drops to one of three print positions. Deflection of the drops is in a direction which is oblique to the direction of head movement. The orifices are positioned in a row perpendicular to the direction of head movement.
The concept of increasing image resolution by increasing the number of print positions serviced by a single ink jet is also shown in U.S. Pat. No. 3,813,676, issued May 28, 1974, to Wolfe; U.S. Pat. No. 3,769,631, issued Oct. 30, 1973, to Hill et al; and U.S. Pat. No. 3,298,030, issued Jan. 10, 1967, to Lewis et al. These patents show print arrangements in which a single jet prints an entire line of characters as the print web is moved past the jet.
Another problem which has become increasingly severe as inter-jet distance has been reduced is that of cross talk between charge electrodes. In systems in which drops are selectively charged, it is important that such charging be accomplished accurately. One source of inadvertent charging is the drops which have previously been formed in the drop streams. Assuming a drop in a stream carries a charge, the subsequent drop in the stream will be formed in sufficient proximity to the charged drop that a slight charge of opposite polarity may be induced. Such drop to drop interference has been recognized as a significant problem and has been treated in several patents such as U.S. Pat. No. 3,828,354, issued Aug. 6, 1974, to Hilton; U.S. Pat. No. 3,512,173, issued May 12, 1970, to Damouth; U.S. Pat. No. 3,827,057, issued July 30, 1974, to Bischoff et al; U.S. Pat. No. 3,789,422, issued Jan. 29, 1974, to Haskell et al; U.S. Pat. No. 3,833,910, issued Sept. 3, 1974, to Chen; and U.S. Pat. No. 3,631,511, issued Dec. 28, 1971, to Keur.
In U.S. Pat. No. 3,656,171, issued Apr. 11, 1972, to Robertson, the problem of cross talk between adjacent jets is also recognized. Inter-jet cross talk occurs when the charge electrodes associated with one jet affect the charging of drops in adjacent jets. In the Robertson U.S. Pat. No. 3,656,171 patent the nature of the device disclosed is such that the effect of the cross talk is minimized and no compensation is provided. Minimization of cross talk is accomplished by requiring each charged drop to induce a charge in an adjacent deflection plate, with the induced charge setting up a weak deflection field. Although reducing substantially the effect of inaccuracy in drop charging, the deflection arrangement of the Robertson U.S. Pat. No. 3,656,171 device is limited in the amount of deflection it can provide. The deflection plate must be positioned extremely close to the jets with the result that dimensional variations become undesirably critical.
In U.S. Pat. No. 3,604,980, issued Sept. 14, 1971, to Robertson, inter-jet cross talk is recognized as a problem with the suggested solution being an increase in shielding between charging electrodes. As the inter-jet distance is reduced, however, such shielding becomes less effective.
There is a need, therefore, for an ink jet recording device which provides substantially increased print density and, further, which reduces errors resulting from drop charging inaccuracies.